Some castings having a hollow structure, such as blades for a gas turbine (turbine blade) made of a Ni-based heat resistant alloy, include hollow cooling holes inside the blade having a complicated shape and formed with high precision in order to increase cooling effect. Such blades can be produced by a lost wax precision casting method or the like by using a ceramic core of a shape corresponding to the intended hollow cooling hole.
The ceramic core is exposed to a molten metal at a temperature of about 1500° C. for several hours during a casting process. Thus, the core is sometimes thermally deformed by the molten metal or deformed by buoyant force, or it is damaged by a flow of the molten metal. Therefore, the ceramic core is required to have a mechanical strength at a high temperature around 1500° C. and dimensional stability so as not to contract or deform at the casting temperature. Moreover, the core is solved to be removed, with use of an aqueous sodium hydroxide or the like after completion of casting and therefore it is required to have dissolvability to the alkaline aqueous solution.
As such a ceramic core, for example, Patent Literature 1 proposes a ceramic core including 60 to 85 mass % of fused silica, 15 to 35 mass % of zircon and 1 to 5 mass % of cristobalite. It states that the ceramic core has a sufficient mechanical strength even at a casting temperature of about 1500° C. and has excellent dimensional stability since remarkable dimensional change is suppressed during a casting process, and that the core is easily dissolved after the casting.
Moreover, Patent Literature 2 proposes a ceramic core produced from 60 to 80 mass % of a fused silica powder, up to 15 mass % of yttria and up to 0.2 mass % of an alkaline metal. It is proposed that the core has a maximum strength of 12 MPa at room temperature and it is not deformed even at a high temperature of 1675° C., which is considerably higher than 1500° C.
Patent Literature 3 proposes a ceramic core produced from fused silica and sodium stabilized colloidal silica. It is proposed that the core has strength of about 7 MPa at a room temperature.